In order to cope with tremendously growing wireless data traffic in recent years, wireless operators and equipment manufacturers have been exploring different options to address high traffic areas with more bandwidth. One option is adapting Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) to operate in the unlicensed band, which combines the robust operation benefits of LTE with the large amount of additional available bandwidth. Furthermore, the system can be controlled by one core network and thus requires low operational expenses.
However, existing technologies other than LTE are already operating in the unlicensed band. It is therefore necessary to devise coexistence solutions to ensure fair sharing among LTE and other wireless technologies in the unlicensed band.
LTE Overview
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT) spread OFDM (also referred to as Single Carrier Frequency Division Multiple Access (SC-FDMA)) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the downlink.
As illustrated in FIG. 2, in the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds (ms), each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each OFDM symbol is approximately 71.4 microseconds (μs).
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled; that is, in each subframe, the base station transmits control information about to which terminal's data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3, or 4 OFDM symbols in each subframe and the number n=1, 2, 3, or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of, e.g., the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
The reference symbols shown in FIG. 3 are the Cell specific Reference Symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
Carrier Aggregation
The LTE Release 10 standard (and subsequent releases) supports bandwidths larger than 20 Megahertz (MHz). One important requirement on LTE Release 10 is to assure backward compatibility with LTE Release 8. This backward compatibility should also include spectrum compatibility. That would imply that an LTE Release 10 carrier that is wider than 20 MHz should appear as a number of LTE carriers to an LTE Release 8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular, for early LTE Release 10 deployments, it can be expected that there will be a smaller number of LTE Release 10-capable terminals compared to many LTE legacy terminals. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Release 10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Release 10 terminal can receive multiple CCs, where the CCs have, or at least the possibility to have, the same structure as a LTE Release 8 carrier. CA is illustrated in FIG. 4. A CA-capable User Equipment device (UE) is assigned a Primary Cell (PCell), which is always activated, and one or more Secondary Cells (SCells), which can be activated or deactivated dynamically.
The number of aggregated CCs as well as the bandwidth of the individual CCs may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal. A terminal may, for example, support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
A UE is first connected to the LTE system via the PCell. The network can then configure the UE to aggregate additional SCell(s). The SCell configuration for activation and deactivation is carried out via Radio Resource Control (RRC) signaling, which typically takes tens of milliseconds.
A configured SCell can be further put into one of two states, namely, activated or deactivated. For an activated SCell, the UE monitors the SCell in order to maintain time/frequency synchronization, monitor control channels, and report Channel Quality/State Information (CQI/CSI) back to the network. For a deactivated SCell, the UE might not monitor the SCell. The activation and deactivation commands are sent by Medium Access Control (MAC) elements, and the UE can apply these commands quickly.
Wireless Local Area Network
In typical deployments of a Wireless Local Area Network (WLAN), Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is used. This means that the channel is sensed, and only if the channel is declared as Idle, a transmission is initiated. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is found Idle. When the range of several Access Points (APs) using the same frequency overlap, this means that all transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP which is within range can be detected. Effectively, this means that if several APs are within range, they will have to share the channel in time, and the throughput for the individual APs may be severely degraded. A general illustration of the Listen-Before-Talk (LBT) mechanism is shown in FIG. 5.
Licensed Assisted Access to Unlicensed Spectrum Using LTE
The spectrum used by LTE is dedicated to LTE. This spectrum dedication has the advantage that the LTE system does not need to care about the coexistence issue and the spectrum efficiency can be maximized. However, unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, when using unlicensed spectrum, LTE would need to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi).
In the License Assisted Access (LAA) framework, as shown in FIG. 6, a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. In this application, a SCell in unlicensed spectrum is denoted as a LAA SCell. Under this LAA framework, the PCell retains the mobility and high layer control while the LAA SCell is geared towards serving the best effort user data. The PCell is supposed to also serve UEs when the unlicensed SCell is unavailable.
Therefore, systems and methods for enabling sharing of unlicensed spectrum between LTE and other wireless technologies (e.g., Wi-Fi) are desired.